The invention provides a process for coating a cylindrical carrier structure with a coating suspension. In particular, the invention provides a process for coating carrier structures for catalysts, for example car exhaust gas catalysts.
The carrier structures for car exhaust gas catalysts are cylindrical with two end faces and an encasing face and a number of flow channels for the exhaust gases of the internal combustion engines which are parallel to the cylinder axis running from the first end face to the second end faces. The carrier structures are also called honeycomb structures.
The cross-sectional shape of the carrier structures depends on the requirements for building it into the vehicle. Carrier structures with a round, elliptical or triangular cross-section are widely used. The flow channels mostly have a square cross-section and are arranged in a tight grid over the entire cross-section of the carrier structure. Depending on the particular application, the channel density or cell density of the flow channels varies between 10 and 140 cmxe2x88x922. Honeycomb structures with cell densities up to 250 cmxe2x88x922 and higher are under development.
Catalyst carrier structures which are obtained by the extrusion of ceramic materials are largely used for the treatment of car exhaust gases. Alternatively, catalyst carrier structures consisting of corrugated and rolled up metal foils are available. Currently, ceramic carrier structures with cell densities of 62 cmxe2x88x922 are still used extensively for the treatment of exhaust gases in private motor vehicles. In this case, the cross-sectional dimensions of the flow channels are 1.27xc3x971.27 mm2. The thickness of the walls in such carrier structures is between 0.1 and 0.2 mm.
In general, very finely divided platinum group metals are used, the catalytic effect of which can be modified by compounds of base metals, to convert the harmful substances present in car exhaust gases, such as carbon monoxide, hydrocarbons and nitrogen oxides, into harmless compounds. These catalytically active components have to be deposited onto the carrier structures. However, it is not possible to ensure the required extremely fine distribution of catalytically active components by the deposition of these components on the geometric surface areas of the carrier structures. This applies equally to non-porous metallic and to porous ceramic carrier structures. A sufficiently large surface area for the catalytically active components can only be made available by applying a support layer made of finely divided high surface area materials to the internal faces of the flow channels. This process is called coating the carrier structure. Coating the outer encasing face of the carrier structures is undesirable and should be avoided in order to avoid the loss of valuable catalytically active materials.
A suspension of the finely divided high surface area materials in a liquid phase, usually water, is used to coat the carrier structures. Typical coating suspensions for catalytic applications contain, as high surface area support materials for the catalytically active components, for example active aluminum oxides, aluminum silicates, zeolites, silicon dioxide, titanium oxide, zirconium oxide and oxygen-storing components based on cerium oxide. These materials form the solids fraction of the coating suspension. In addition, soluble precursors of promoters or catalytically active noble metals from the platinum group in the Periodic System of Elements may also be added to the coating suspension. The solids content of typical coating suspensions is in the range between 20 and 65 wt. %, with respect to the total weight of the suspension. The density is between 1.1 and 1.8 kg/l.
A number of processes for depositing the support layer on the carrier structures using a coating suspension or slurry is known from the prior art. For coating purposes, the carrier structures, may be for example immersed in the coating suspension or the coating suspension may be poured over the carrier structures. Furthermore, there is the possibility of pumping or sucking the coating suspension into the channels in the carrier structures. In all cases, excess coating material has to be removed from the channels in the carrier structure under suction or by blowing out with compressed air. Any channels blocked with coating suspension are opened up by this means.
After the coating procedure, the carrier structure and support layer are dried and then calcined to solidify and fix the support layer to the carrier structure. Then the catalytically active components are introduced to the coating by impregnating with mostly aqueous solutions or precursor compounds of the catalytically active components. As an alternative, the catalytically active components may also be added to the coating suspension itself. Subsequent impregnation of the final support layer with catalytically active components is not required in this case.
An essential criterion for the coating process is the coating or loading concentration which can be achieved therewith in one working stage. This is understood to be the proportion of solids which remains on the carrier structure after drying and calcining. The coating concentration is given in grams per liter volume of the carrier structures (g/l). In practice, coating concentrations of up to 300 g/l are required in car exhaust gas catalysts. If this amount cannot be applied in one working stage with the process used, then the coating procedure has to be repeated, after drying and optionally calcining the carrier structure, often enough to achieve the desired loading. Frequently, two or more coating procedures using coating suspensions of different composition are performed. Catalysts which have several superimposed layers with different catalytic functions are obtained in this way.
Another criterion for the quality of a coating is its uniformity, in both the radial and axial direction of the carrier structure. Irregularities in the axial direction cause particular problems because they can lead at high target loadings to pressure losses which can no longer be tolerated.
It is known in the art that one way to achieve uniformity in the coating of a catalyst carrier structures in honeycomb form also called honeycomb structures is to vertically align the cylindrical axis of the honeycomb structure so the coating suspension is pumped into the channels through the lower end face of the honeycomb structure until it emerges from the upper end face. Then, the coating suspension is again pumped out downwards and excess coating suspension is removed from the channels by blowing out or under suction in order to avoid blocking the channels. Using this process, support layers are obtained which have high uniformity over the entire length of the honeycomb structure.
Other ways to coat ceramic honeycomb structures are known in the art. For example, a previously determined amount of a coating suspension is placed in a flat vessel and one end face of the honeycomb structure to be coated is dipped into the suspension. The previously determined amount of coating suspension corresponds to the target amount of coating for the honeycomb structures. Then the entire amount of coating suspension is pulled into the flow channels of the honeycomb structure under suction by applying a vacuum to the second end face. Since the previously determined amount of coating suspension corresponds to the target amount of coating for the honeycomb structures, no removal of excess coating suspension from the flow channels is required after introducing the coating suspension under suction. Coating is preferably performed in two steps, wherein in a first step 50 to 85% of the amount of coating required is introduced to the flow channels under suction from the first end face and the remaining amount of coating is introduced from the second end face of the honeycomb structure. This process results in a high degree of reproducibility for the coating concentration. However, the catalysts produced in this way exhibit a steep gradient in the thickness of the coating along the honeycomb structure. Also, the preferred method for coating the honeycomb structure in two steps does not sufficiently improve the uniformity of the coating along the honeycomb structure.
It is known in the art that for certain applications, catalysts are required which have different catalytically active regions along the catalyst carrier structure. For example, a catalyst which consists of two partial catalysts, a catalyst at the inflow end for the selective catalytic reduction of nitrogen oxides by ammonia or an ammonia-donating compound and an oxidation catalyst at the outflow end, wherein the oxidation catalyst is applied as a coating to the section at the outflow end of a one-piece reduction catalyst specified as a full extrudate in honeycomb form and the outflow section makes up 20 to 50% of the total catalyst volume. Application of the oxidation catalyst is performed by immersing the desired length of the outflow end of the honeycomb structure in the coating suspension for the oxidation catalyst.
The prior art also proposes reinforcing the end faces of monolithic catalysts for exhaust gas treatment by the application or incorporation of inorganic substances which reinforce the mechanical properties of the carrier structures or catalytic coating. The length of the reinforced zone, starting from the end face concerned, is up to twenty times the diameter of a channel. To perform this coating procedure, it is suggested that the catalyst structures be immersed in a suspension of the reinforcing substance or that this suspension be sprayed onto the end faces of the structures.
A process for the partial coating of honeycomb structures or carrier structures has also been proposed. For coating purposes, one end face of the substrate is immersed in a bath containing the coating suspension. The bath contains more coating suspension than the amount to coat the substrate up to a desired height. Then a reduced pressure is applied to the second end fact, the strength and duration of this being sufficient to draw the coating suspension in the channels up to the required height. Efforts are made to achieve the same coating in all the channels.
This process has several distinct disadvantages. The height of the coating and its axial length is determined by the use of capillary forces and the size of the reduced pressure applied and by the time during which the reduced pressure is applied to the second end face of the carrier structure. Periods of 1 to 3 seconds are cited for this. Thus, changes in the viscosity of the coating suspension lead to direct changes in the length of coating applied. The strength of the reduced pressure applied is a maximum of one inch of a water column, which corresponds to about 2.5 mbar. Accurate control of this small reduced pressure is also difficult and can lead to further problems with the reproducibility of the coating process. Due to the low reduced pressure, only coating suspensions with a low viscosity can be processed using this method, which means that these suspensions generally have only a low solids content. Again, the low solids content means that several coating procedures have to be performed in sequence in order to apply a high loading concentration. In this process capillary forces play an essential part. This makes the process dependent on the cell density of the carrier structures being coated.
Based of the foregoing, there is a need in the art for a process for coating carrier structures with coating suspensions having a high solids content that ensure high uniformity of the coating thickness in the axial direction of the carrier structure.
The present invention provides a process for coating a cylindrical carrier structure with a predetermined amount (target take-up) of a coating suspension, wherein the carrier structure has a cylinder axis, two end faces, an encasing face and an axial length L and a number of channels running from the first end face and the second end face. The process comprises the following process steps:
a) vertically aligning the cylinder axis of the carrier structure and filling the empty volume of the channels up to a predetermined height H1 starting from the lower end face;
b) removing the excess coating suspension through the lower end face of the carrier structure down to the target take-up;
c) turning the carrier structure 180xc2x0, so that the upper and lower end faces are exchanged one for the other; and
d) repeating steps a) and b), wherein the height H2, up to which the channels are filled is given by H2=Lxe2x88x92xxc2x7H1 where x is between 0.8 and 1.0.
The proposed process thus includes two coating steps or coatings which are introduced into the channels from opposite end faces of the carrier structure.
Target take-up in the context of this invention is understood to be the amount of coating suspension which is meant to remain on the structure after the completion of coating. Since the process includes two coating steps, the target take-ups for the first and second coating steps have to be differentiated. The two values together give the target take-up for the entire coating process.
For a better understanding of the present invention together with other and further advantages and embodiments, reference is made to the following description taken in conjunction with the examples, the scope of which is set forth in the appended claims.